Digital trigger filter for a real time digital oscilloscope

Abstract

A real time DSO is equipped with a Digital Trigger Filter that performs high frequency rejection, low frequency rejection, AC and DC triggering. The Digital Trigger Filter includes first and second digitally implemented IIR (Infinite Input Response) Filters. A digitized Conditioned Input Signal is applied to the first IIR Filter. It has taps that provide the Trigger Signal outputs needed for high and low frequency rejection. The high frequency rejection output of the first ER Filter is essentially a low pass output (3 dB down at 50 KHz) and is also used as the digital input to the second IIR Filter, whose output is a much more aggressive suppression of high frequencies (3 dB down at 50 Hz). The AC Trigger Signal output is produced by subtracting the output of the second IIR filter from the original input to the entire Digital Trigger Filter, and the DC Trigger Signal output is simply the same as that original input. A MUX selects which Trigger Signal is applied to a Digital Trigger Comparator.

Description

BACKGROUND OF THE INVENTION [0001] DSOs (Digital Sampling Oscilloscopes) can be divided into two broad classifications: those that operate using ‘real time’ and those that operate using ‘equivalent time.’ A real time DSO takes consecutive samples at a rate high enough to meet Nyquist criteria for even ‘single shot’ events that contain frequency components up to the bandwidth of the ‘scope. And although a real time DSO will operate perfectly fine with repetitive inputs, there is no requirement that the input be repetitive to achieve full bandwidth. An equivalent time DSO cannot sample at the Nyquist rate, and relies upon the repetitive nature of the input to accumulate a replica of the input by sampling at a much lower rate, but at successively further locations along the input waveform, relative to a selected location identified by a trigger event. [0002] Today, virtually all oscilloscopes are triggered, in contrast to the original technique of simply synchronizing a free-running sw...

AI Technical Summary

Benefits of technology

[0007] A digitized Conditioned Input Signal is applied to the first IIR Filter. It has taps that provide the Trigger Signal outputs needed for high and low frequency rejection. The high frequency rejection output of the first IIR Filter is essentially a low pass output (3 dB down at 50 KHz) and is also used as the digital input to the second IIR Filter, whose output is a much

Problems solved by technology

It is made more complicated when the signal being observed contains many different components, or types of behaviors.

They are implemented with actual physical circuitry, and as such are limited in their range of operation by the dictates of circuit design and its compromises, they take space and generate heat, require adjustment, are susceptible of failure, and they cost money.

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